The concentration of a given gas may be important in many different fields. One such field is the use of medical gases, where the correct concentration may be critical from the point of view of the efficacy and/or safety of administration of the gas to a patient. For example, nitrous oxide is used in admixture with oxygen, typically as a 50/50 mixture, for short anaesthetic applications. It is extremely good, giving almost immediate pain relief, and flushes out of the body very quickly. In some countries the use of a pre-mixed gas is allowed, which permits a mixture of precisely known proportions to be used. In other countries, notably the USA, the use of pre-mixed gas is not permitted. In those circumstances, oxygen and nitrous oxide can be supplied separately and mixed on demand in a mixing device. It is important, however, that the correct proportions of the gases are mixed, the departure of the proportion of oxygen from the target level of 50%, according to standards, being limited to +/−5%. Furthermore, in the event of the proportion of oxygen falling below 20% (known as a hypoxic mix), the use of the oxygen-deficient mixture may result in hypoxia, posing a serious risk to the patient's health.
One device that is widely used to match supply of a gas to inhalation is a demand valve, in which the valve opens and closes in response to inhalation by the patent. A number of ways of mixing gases and delivering to a patient on demand are known. A problem encountered with such arrangements is that, in the case of a failure of a gas mixing device resulting in the mix being outside the range set by standards or, in a particularly serious case, being a hypoxic mix, it is not possible to detect the resultant incorrect flows of N2O and/or O2.
Inline flow indicators are widely used to monitor flow of gases. The flow passes a flow-meter which provides a visual indication of the flow of the gas. Such systems rely upon the vigilance of staff for appropriate action to be taken in the event that the indicator shows a malfunction. Flow-meters are sensitive at low flows of the kind relevant in the medical field, and can indicate leaks. However, especially where used with a valve they are relatively costly, and dynamic movement, for example of the patient receiving the gas, may make it difficult to detect flow during use. Flow-meters can, moreover, be orientation-sensitive, casting doubt upon the reliability of readings, for example if a device is portable and is stood on an uneven surface. “Popping indicators” (indicators that indicate when flow is over a given threshold) will only detect failures reliably when the indicators do not appear on inhalation by the patient. Additionally, it is not straightforward to use flow-meters to generate an audible alarm, or to prompt automatic remedial adjustment of the gas supply, either or both of which would be advantageous.
It is known to monitor the concentration of gases using electrochemical or paramagnetic devices. The known monitors are complex and expensive, requiring in at least some cases a power supply, appropriate software algorithms, and interfaces to other components, whilst further suffering from the disadvantages of need for calibration to achieve reliable readings and battery life. The complexity of the known monitors adds to cost and can make implementation and obtaining regulatory approval more complicated.
There is a need for a device which allows the ratio of two or more gases in a mixture to be monitored simply and reliably in order that any departure from a desired ratio can be recognised. Further, it would be desirable for the device to be compact in order that it can be housed within a small container and is easy to carry. It would be desirable for such a device to give a signal that would be used for either or both of generating an audible alarm and prompting adjustment (which may include switching off) of the gas supply.